The Male-Female Gap in Marathoning: Physiology or Participation?

There's an interesting paper, just published ahead of print in Medicine & Science in Sports & Exercise, by researchers at Marquette that analyzes sex differences in New York Marathon performances between 1980 and 2010. You can read the abstract here, or look at the following pictures to get a brief glimpse of the story the data tells:

This is the first key observation, showing the average gap between first place and subsequent top-10 placings, averaged over all five-year age groups between 20 and 79. You can see that the gap widens more quickly for women than men, an indication that the female also-rans tend to be farther behind the leaders.

These next two graphs tell us two things: first, the depth difference between women and men has been narrowing over the years. Second, the difference gets bigger and bigger as you go to older age groups. So what does this mean? Is there some physiological reason that women get farther behind and have less competitive depth as they get older? Here's the explanation the researchers come up with:

This graph shows that, in general, the lack of depth shows up when the total number of finishers in a given age category is lowest. This isn't too surprising: I think most observers would agree that the apparent difference between male and female performance, especially when you move away from the winners, is exaggerated by the relatively higher male participation. Still, it's neat to see it teased out statistically like this.